This invention relates generally to electrical energy conversion and, more specifically, to control of multiphase power converters used for electrical energy conversion.
With the rising cost and scarcity of conventional energy sources and concerns about the environment, there is a significant interest in alternative energy sources such as solar power generation systems, fuel cells, wind turbines, and marine hydro kinetic devices. Most of these alternative energy sources utilize three phase pulse width modulated (PWM) converters to transmit electricity to a power grid.
Three-phase PWM converters connected to the power grid are typically regulated in a current control mode wherein controllers use voltages and currents of the power grid and phase locked loop (PLL) circuits to obtain control signals to provide the phase and the frequency for the currents to be injected into the grid. However, if the grid is not available, as is the case when an alternative energy source is operating in an island mode, the converters are forced to leave the current control mode and switch to a voltage control mode wherein the frequency and phase of the voltage is generated internally. In such cases, the controllers generally include current limiters at their outputs to limit the output currents and are designed to have enough bandwidth to generate balanced voltages under operating conditions with unbalanced loads. The design and tuning of these limiters is rather cumbersome due to the non-linear nature of the limiters and due to the variable nature of the loads.
Further, these converters are connected to the grid or loads through transformers in such a manner that the controllers are designed to provide control signals in a manner to avoid operating modes that could bring the cores of these transformers into saturation. Another challenge is that it is not easy to execute a smooth transition from current control to voltage control when the grid is not available or from voltage control back to current control when the grid becomes available again.
Therefore, it is desirable to provide a method and a system that will address the foregoing issues.